Do not microwave glass in the same microwave you use for food - the colors in glass are made using metals like cadmium, copper, chromium, uranium, selenium, arsenic and so on.
Casting in your home - buy a cheap microwave. The microwave I use is about 30 years old - it is really nasty inside (
pic of nastiness here ). I have been using this machine for more than 5 years and (against all instructions) I keep the glass turntable to fight against hot-spots. If the turntable breaks, you can use the glass in one of your projects later but 5+ years tells me not to worry about it.
I started by using found glass - there is a cheap, nasty beer that comes in a beautiful blue bottle (also, a vodka); someone broke out the window in my car, I used that glass to cast a 18th cent. mounted Prussian soldier. I also use the glass stones found in hobby shops (NOT h*bby/l*bby since they are anti-lgbtq) which runs about $1.00 per pound. Lately, I have been spending $60/kilo on glass from a high-end glass supplier but that is just me (and compatibility is important when mixing colors).
Which brings me to the last important rule: glass compatibility - glass expands when it is heated and then contracts as it cools. Glass is rated on its COE coefficient of expansion so if you mix glass with different COEs some of the glass within the melt will shrink more than the other creating an internal tension that generally cracks the glass as it approaches room temperature. Cooling the glass object too fast will also generate internal stresses that will cause cracking (I have a piece that I made 14 years ago that has a stress fracture internally about an inch long - I expect in 40 years it will shatter :). The glass in bottles (called soda glass) is different from glass in windows (called float glass since it floats on molten tin) do not mix the 2 (and do not recycle float glass, they do not want it).
I have been meaning to write an indestructible for instructibles.com so if anyone is interested, I could practice here.
Here I am working with The Mandalorian (better known to older folk as Bobba Fett)
I am highlighting one of the features of his helmet using midnight blue powdered glass applied using that squeeze bottle up top.
Then I filled the mold with crystal-clear glass
(damn that pic sucks - I’ll fix it in post) - I try to
gendercolor-bend my work like bright blue skulls - that sort of thing. (I ‘randomly’ gave a neighbor a clear glass Bobba Fett and she immediately recognized it as The Mandalorian). Here is it glowing (trying for a better pic, I over-nuked the piece and the blue started to migrate). It will be an hour or so before I can decant the piece so that will have to be a different post.